Help the museum curator complete the exhibition label for a bizarre artifact.

Any artifacts _____ to the notorious pirate Captain Blackbeard must be handled with extreme care, as they are most certainly cursed.

The correct answer is belonging.

This is an active reduced relative clause. The original clause is "which belong to...". To reduce an active relative clause, we replace the verb with its present participle (-ing) form.

Even though "belong" is a stative verb that isn't normally used in continuous tenses (we don't say "are belonging"), it must take the -ing form when used in a reduced relative clause. "Belonged" is incorrect because "belong" cannot be passive.